Field of the Invention
The aspect of the embodiments relates to image forming apparatuses which correct distortion and density unevenness of a 2D image at a time of image formation, such as digital copiers, multifunction devices, and laser printers.
Description of the Related Art
As an electrophotographic method of an image forming apparatus, such as a laser printer or a copier, a method for forming a latent image on a photoreceptor using an optical scanning device which performs scanning using laser light is generally employed. In such an optical scanning device employing a laser scanning method, laser light formed in parallel light using a collimator lens is deflected by a rotatable polygonal mirror and an image is formed on the photoreceptor using the deflected laser light through a long fθ lens. Furthermore, such an optical scanning device employs a multi-beam scanning method for performing scanning simultaneously using a plurality of laser beams emitted from a multi-beam light source having a plurality of light emitting elements in one package.
On the other hand, to form an excellent image which does not include density unevenness or banding, in one embodiment, scanning lines of laser light are arranged at regular pitches on the photoreceptor. However, the pitches between the scanning lines may vary due to a plurality of reasons below. For example, the pitches between the scanning lines vary due to variation of a surface speed of the photoreceptor, variation of a rotation speed of the rotatable polygonal mirror, or the like. Furthermore, the pitches between the scanning lines also vary due to variation of an angle of a mirror surface of the rotatable polygonal mirror relative to a rotation axis of the rotatable polygonal mirror or variation of pitches between light emitting points arranged on a laser chip in a case of the multi-beam light source. In FIG. 16A, scanning with laser light is denoted by horizontal lines and a state in which pitches between the scanning lines periodically vary is illustrated. As illustrated in FIG. 16A, development is performed with high density in a case where a pitch between the scanning lines of the laser light is small whereas development is performed with low density in a case where a pitch between the scanning lines of the laser light is large, and accordingly, a stripe pattern (moire) is likely to be detected. To address such density unevenness and banding caused by the reasons described above, a technique of correcting banding by controlling an exposure amount of the optical scanning device has been proposed. For example, Japanese Patent Laid-Open No. 2012-098622 discloses a configuration in which a beam position detection unit for a sub scanning direction is disposed in the vicinity of a photoreceptor and an exposure amount of an optical scanning device is controlled based on scanning pitch information obtained from detected beam positions so that banding becomes unnoticeable.
Furthermore, an image forming apparatus performs a halftone process on image data using a dither pattern so that a halftone (intermediate gradation) is expressed. A line screen or a dot screen, for example, is used for an image which is subjected to the halftone process.
However, some screens used in the halftone process are affected by tilt of the mirror surface of the rotatable polygonal mirror (hereinafter simply referred to as “plane tilt” of a rotatable polygonal mirror) and others are not. FIGS. 16B and 16C are diagrams illustrating a phenomenon of the plane tilt of the rotatable polygonal mirror. In FIGS. 16B and 16C, gray portions denote dither patterns. Furthermore, light gray portions (white portions) denote portions in which a pitch between scanning lines of laser light emitted from a light source is sparse, and dark gray portions (black portions) denote portions in which a pitch between scanning lines is dense. In an image using a line screen illustrated in FIG. 16B, a stripe pattern of the line screen regularly extends across portions where dense/sparse portions of the scanning lines are generated, and therefore, moire is emphasized. On the other hand, in an image using a dot screen illustrated in FIG. 16C, when compared with the case of the line screen, portions where dots and sparse/dense portions overlap with each other are irregularly generated, shades of gray are less generated when compared with the case of the line screen, and a degree of moire is lower when compared with the case of the line screen.
Furthermore, in a case where an exposure amount is controlled when the density unevenness caused by dense/sparse portions of the scanning lines is corrected, since density per a predetermined area is not stored before and after the correction, the correction may not appropriately function depending on an input image pattern, and accordingly, correction performance may be degraded. Here, FIGS. 17C and 17D are diagrams illustrating correction performed by extracting a portion of an image pattern (the line screen) of FIG. 16A using a general method for performing density adjustment using an exposure amount as disclosed in Japanese Patent Laid-Open No. 2012-098622. Specifically, FIG. 17C is a diagram illustrating an image pattern before the correction, and FIG. 17D is a diagram illustrating an image pattern after the correction. Furthermore, “A1” and “A2” of FIGS. 17C and 17D indicate correction target ranges, and “B1” and “B2” of FIG. 17D including the correction target ranges A1 and A2, respectively, indicate ranges which have been subjected to the correction. In FIG. 17D, image density is corrected in the correction target ranges A1 and A2 when compared with FIG. 17C. However, in the ranges B1 and B2 including surrounding portions of the correction target ranges A1 and A2, portions of high image density and portions of low image density are generated, that is, excessive correction occurs, since the method does not store density before and after the correction, and accordingly, correction may fail depending on a pattern of an input image.
To address this situation in the general method, an exposure method for correcting dense/sparse portions by shifting a center of density over a plurality of pixels as illustrated in FIGS. 18A to 18C is considered. However, use of the method for shifting a center of density may not obtain a correction effect if the shifted density may not be accurately reproduced in accordance with a gradation characteristic. Furthermore, environmental variation, such as aging, variation in temperature, or variation in humidity, considerably affects the gradation characteristic of electrophotography. Therefore, appropriate correction is applied when the gradation characteristic varies due to the environmental variation.